Semiconductor lasers have found widespread use in modern technology as the light source of choice in, e.g., communications systems, compact disc players, etc. The typical semiconductor laser is a double heterostructure with a narrow bandgap, high refractive index layer surrounded on opposed major surfaces by wide bandgap, low refractive index layers. The low bandgap layer is termed the "active layer", and the bandgap and refractive index differences serve to confine both charge carriers and optical energy to the active layer or region. Opposite ends of the active layer have mirror facets which form the laser cavity. The cladding layers have opposite conductivity types and when current is passed through the structure, electrons and holes combine in the active layer to generate light.
While perfectly adequate for many applications, the structure described has two drawbacks which are pertinent to this invention. First, the active layer has a longitudinal axis which runs parallel to the major surfaces of the structure and substrate and the light is emitted in a direction generally parallel to the longitudinal axis; i.e., the structure is an edge emitting device. However, for many purposes, it is desirable that light be emitted in a direction perpendicular to the substrate, i.e., that the device be a surface emitting device. For example, surface emitting devices can be fabricated in arrays with relative ease while edge emitting devices can not be so fabricated. Such arrays are potentially useful in such diverse applications as, for example, image processing inter-chip communications, i.e., optical interconnects, etc. Second, the laser is typically turned ON and OFF by varying the current flow through the device. This requires a relatively large change in the current through the device which is undesirable.
Several types of surface emitting lasers have been developed. One such laser of special promise is termed a "vertical cavity surface emitting laser". See, for example, Optical Engineering, 29, pp. 210-214, March 1990 for a description of this laser. Also, see Electronics Letters, 26 pp. 710-711 , Mar. 24, 1990. The laser described has an active region with one or more quantum well layers. The quantum well layers are interleaved with barrier layers. On opposite sides of the active region are mirror stacks which are formed by interleaved semiconductor layers having properties, such that each layer is typically a quarter wavelength thick at the wavelength (in the medium) of interest thereby forming the mirrors for the laser cavity. There are opposite conductivity type regions on opposite sides of the active region, and the laser is turned ON and OFF by varying the current through the active region. Variations, including dielectric and metal mirrors and metal contacts, have been demonstrated.
An array of lasers is fabricated by growing the desired layers on a substrate and then patterning the layers to form the array. Individual lasers may be separately contacted with appropriate contacts. Laser densities in excess of a million lasers per square centimeter can presently be obtained.
However, a technique for turning the laser ON and OFF and for varying the intensity of the emitted radiation from a vertical cavity surface emitting laser which is easily monolithically integrated with the laser would be desirable.